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The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8 epitaxial thin films grown by molecular-beam epitaxy (MBE) exhibit emergent 2D ferromagnetism with intrinsic spin polarization of the V 3d electrons despite that the bulk counterpart is 'originally antiferromagnetic (AFM)'. Moreover, thickness-dependence measurements reveal that this newly-developed 2D ferromagnet could be classified as an itinerant 2D Heisenberg ferromagnet with weak magnetic anisotropy, broadening a lineup of 2D magnets to those potentially beneficial for future spintronics applications.

Magnetic skyrmions are the self-organized topological spin textures behaving like particles. Because of their fast creation and typically long lifetime, experimental verification of skyrmion's creation/annihilation processes has been challenging. Here we successfully track skyrmions dynamics in defect-introduced Co9Zn9Mn2, by using pump-probe Lorentz transmission electron microscope. Following the nanosecond-photothermal excitation, we resolve 160-nm-skyrmion's proliferation at, 14 pages, 5 figures, Supplementary materials

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe$_2$, that hosts a charge density wave (CDW) coupled with the band inversion involving V3$d$ and Te5$p$ orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli., Comment: 46 pages, 6 main figures, 12 supplementary figures

(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature $({T}_{\mathrm{C}})$ is above room temperature. However, the origin of ferromagnetism with high ${T}_{\mathrm{C}}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ including the $\mathrm{Fe}\text{\ensuremath{-}}3d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The $\mathrm{Fe}\text{\ensuremath{-}}3d$ resonant ARPES spectra demonstrate that the $\mathrm{Fe}\text{\ensuremath{-}}3d$ IB crosses the Fermi level $({E}_{\mathrm{F}})$ and hybridizes with the VB of GaSb. These observations indicate that the VB structure of $(\mathrm{G}{\mathrm{a}}_{0.95},\mathrm{F}{\mathrm{e}}_{0.05})\mathrm{Sb}$ is consistent with that of the IB model which is based on double-exchange interaction between the localized $3d$ electrons of the magnetic impurities.

This paper establishes the angle dependence of the upper critical field H${}_{c2}$ of a two-dimensional Ising superconductor NbSe${}_{2}$. The experimental results demonstrate the cusp-like behavior around the parallel magnetic fields even at the lowest temperature where H${}_{c2}$ should be determined by the pure paramagnetic effect. Those results are well described by the formula that includes contributions both from the orbital and paramagnetic effects.

GeTe is a chemically simple IV-VI semiconductor which bears a rich plethora of different physical properties induced by doping and external stimuli. These include, among others, ferromagnetism, ferroelectricity, phase-change memory functionality, and comparably large thermoelectric figure of merits. Here we report a superconductor - semiconductor - superconductor transition controlled by finely-tuned In doping. Our results moreover show the existence of a critical doping concentration around $x = 0.12$ in Ge$_{1-x}$In$_{x}$Te, where various properties take either an extremum or change their characters: The structure changes from polarly-rhombohedral to cubic, the resistivity sharply increases by orders of magnitude, the type of charge carriers changes from holes to electrons, and the density of states diminishes at the dawn of an emerging superconducting phase. By core-level photoemission spectroscopy we find indications of a change in the In-valence state from In$^{3+}$ to In$^{1+}$ with increasing $x$, suggesting that this system is a new promising playground to probe valence fluctuations and their possible impact on superconductivity., Comment: Main text (4 Figures) + Supplement(9 Figures)

The competing magnetic ground states of the itinerant magnet EuCuSb, which has a hexagonal layered structure, were studied via magnetization, resistivity, and neutron diffraction measurements on single-crystal samples. EuCuSb has a three-dimensional semimetallic band structure as confirmed by band calculation and angle-resolved photoelectron spectroscopy, consistent with the nearly isotropic metallic conductivity in the paramagnetic state. However, below the antiferromagnetic transition temperature of TN1 (8.5 K), the resistivity, especially along the hexagonal axis, increases significantly. This implies the emergence of anisotropic magnetic ordering coupled to the conducting electrons. Neutron diffraction measurements show that the Eu spins, which order ferromagnetically within each layer, are collinearly modulated (up-up-down-down) along the hexagonal axis below TN1, followed by the partial emergence of helical spin modulation below TN2 (6 K). Based on the observation of anomalous magnetoresistance with hysteretic behavior, we discuss the competing nature of the ground state inherent in a frustrated Heisenberg-like spin system with a centrosymmetric structure., Comment: 12 pages, 4 figures, Physical review B in press

Quantum states characterized by nontrivial topology produce interesting electrodynamics and versatile electronic functionalities. One source for such remarkable phenomena is emergent electromagnetic field, which is the outcome of interplay between topological spin structures with scalar spin chirality and conduction electrons. However, it has scarcely been exploited for emergent function related to heat-electricity conversion. Here we report an unusually enhanced thermopower by application of magnetic field in MnGe hosting topological spin textures. By considering all conceivable origins through quantitative investigations of electronic structures and properties, a possible origin of large magneto-thermopower is assigned to the strong energy dependence of charge-transport lifetime caused by unconventional carrier scattering via the dynamics of emergent magnetic field. Furthermore, high-magnetic-field measurements corroborate the presence of residual magnetic fluctuations even in the nominally ferromagnetic region, leading to a subsisting behavior of field-enhanced thermopower. The present finding may pave a way for thermoelectric function of topological magnets., Topological spin textures produce versatile electronic functionalities, but are scarcely exploited for achieving heat to electricity conversion. Here, Fujishiro et al. attribute an enhanced magneto-thermopower in MnGe with topological spin hedgehogs, to electron scattering via the dynamics of an emergent magnetic field.

The chiral crystal is characterized by a lack of mirror symmetry and an inversion center, resulting in the inequivalent right- and left-handed structures. In the noncentrosymmetric crystal structure, the spin and momentum of electrons are locked in the reciprocal space with the help of the spin-orbit interaction. To reveal the spin textures of chiral crystals, here we investigate the spin and electronic structure in p-type semiconductor elemental tellurium with a chiral crystal structure by using spin- and angle-resolved photoemission spectroscopy. Our data demonstrate that the highest valence band crossing the Fermi level has a spin component parallel to the electron momentum around the BZ corners. Significantly, we have also confirmed that the spin polarization is reversed in the crystal with the opposite chirality. The results indicate that the spin textures of the right- and left-handed chiral crystals are hedgehog-like, leading to unconventional magnetoelectric effects and nonreciprocal phenomena., Comment: 15 pages, 4 figures

In this paper, we investigate the femtosecond-optical-pulse-induced strain dynamics in relatively thin (100 nm) and thick (10 000 nm) silicon plates based on finite-element simulations. In the thin sample, almost spatially homogeneous excitation by the optical pulse predominantly generates a standing wave of the lowest-order acoustic resonance mode along the out-of-plane direction. At the same time, laterally propagating plate waves are emitted at the sample edge through the open edge deformation. Fourier transformation analysis reveals that the plate waves in the thin sample are mainly composed of two symmetric Lamb waves, reflecting the spatially uniform photoexcitation. In the thick sample, on the other hand, only the near surface region is photo-excited and thus a strain pulse that propagates along the out-of-plane direction is generated, accompanying the laterally propagating pulse-like strain dynamics through the edge deformation. These lateral strain pulses consist of multiple Lamb waves, including asymmetric and higher-order symmetric modes. Our simulations quantitatively demonstrate the out-of-plane and in-plane photoinduced strain dynamics in realistic silicon plates, ranging from the plate wave form to pulse trains, depending on material parameters such as sample thickness, optical penetration depth, and sound velocity.

High-temperature superconductivity in iron-pnictides/chalcogenides arises in balance with several electronic and lattice instabilities. Beside the antiferromagnetic order, the orbital anisotropy between Fe 3d xz and 3d yz occurs near the orthorhombic structural transition in several parent compounds. However, the extent of the survival of orbital anisotropy against the ion-substitution remains to be established. Here we report the composition (x) and temperature (T) dependences of the orbital anisotropy in the electronic structure of a BaFe2(As1−xP x )2 system by using angle-resolved photoemission spectroscopy. In the low-x regime, the orbital anisotropy starts to evolve on cooling from high temperatures above both antiferromagnetic and orthorhombic transitions. By increasing x, it is gradually suppressed and survives in the optimally doped regime. We find that the in-plane orbital anisotropy persists in a large area of the nonmagnetic phase, including the superconducting dome. These results suggest that the rotational symmetry-broken electronic state acts as the stage for superconductivity in BaFe2(As1−xP x )2.

Science advances 4(7), eaar3867 - (2018). doi:10.1126/sciadv.aar3867, The observation and control of interweaving spin, charge, orbital, and structural degrees of freedom in materials on ultrafast time scales reveal exotic quantum phenomena and enable new active forms of nanotechnology. Bonding is the prime example of the relation between electronic and nuclear degrees of freedom. We report direct evidence illustrating that photoexcitation can be used for ultrafast control of the breaking and recovery of bonds in solids on unprecedented time scales, near the limit for nuclear motions. We describe experimental and theoretical studies of IrTe%$_2$ using femtosecond electron diffraction and density functional theory to investigate bonding instability. Ir-Ir dimerization shows an unexpected fast dissociation and recovery due to the filling of the antibonding $d_{xy}$ orbital. Bond length changes of 20% in IrTe$_2$ are achieved by effectively addressing the bonds directly through this relaxation process. These results could pave the way to ultrafast switching between metastable structures by photoinduced manipulation of the relative degree of bonding in this manner., Published by AAAS, Washington, DC [u.a.]

Antiferroic electronic instability in Ba,KFe2As2 persists in the nonmagnetic phase covering the superconducting dome., A major problem in the field of high-transition temperature (Tc) superconductivity is the identification of the electronic instabilities near superconductivity. It is known that the iron-based superconductors exhibit antiferromagnetic order, which competes with the superconductivity. However, in the nonmagnetic state, there are many aspects of the electronic instabilities that remain unclarified, as represented by the orbital instability and several in-plane anisotropic physical properties. We report a new aspect of the electronic state of the optimally doped iron-based superconductors by using high–energy resolution angle-resolved photoemission spectroscopy. We find spectral evidence for the folded electronic structure suggestive of an antiferroic electronic instability, coexisting with the superconductivity in the nonmagnetic state of Ba1−xKxFe2As2. We further establish a phase diagram showing that the antiferroic electronic structure persists in a large portion of the nonmagnetic phase covering the superconducting dome. These results motivate consideration of a key unknown electronic instability, which is necessary for the achievement of high-Tc superconductivity in the iron-based superconductors.

The low energy electronic structure of LaFeAsO1-xHx (0.0 < x < 0.60), the system which exhibits two superconducting domes in its phase diagram, is investigated by utilizing the laser photoemission spectroscopy. From the precise temperature-dependent measurement of the spectra near the Fermi level, we find the suppression of the density of states with cooling, namely the pseudogap formation, for all doping range. The pseudogap in the low x range (i.e. the first superconducting dome regime) gets suppressed with increasing x, more or less similarly to the previous results in F-doped LaFeAsO system. On the other hand, the pseudogap behavior in the second superconducting dome regime at high-x becomes stronger with increasing the H-doping level. The systematic doping dependence shows that the pseudogap is enhanced toward the both ends of the phase diagram where the different types of antiferromagnetic order exist., 8 pages, 6 figures

UTokyo Research掲載「バレートロニクス材料の提案」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/a-proposed-valleytronics-material/, UTokyo Research "A proposed valleytronics material" URI: http://www.u-tokyo.ac.jp/en/utokyo-research/research-news/a-proposed-valleytronics-material/

We investigate the surface electronic structures of polar 1T'-MoTe2, the Weyl semimetal candidate realized through the nonpolar-polar structural phase transition, by utilizing the laser angle-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. Two kinds of domains with different surface band dispersions are observed from a single-crystalline sample. The spin-resolved measurements further reveal that the spin polarizations of the surface and the bulk-derived states show the different domain-dependences, indicating the opposite bulk polarity. For both domains, some segment-like band features resembling the Fermi arcs are clearly observed. The patterns of the arcs present the marked contrast between the two domains, respectively agreeing well with the slab calculation of (0 0 1) and (0 0 -1) surfaces. The present result strongly suggests that the Fermi arc connects the identical pair of Weyl nodes on one side of the polar crystal surface, whereas it connects between the different pairs of Weyl nodes on the other side., 13 pages, 4 figures

We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods. For a single-crystalline Au thin film, we observe the suppression of the diffraction intensity occuring in 10 ps, which direcly reflects the lattice thermalization via the electron-phonon interaction. By using the two-temperature model, the electron-phonon coupling constant (g) and the electron and lattice temperatures (Te, Tl) are evaluated from UED, with which we simulate the transient optical transmittance. The simulation well agrees with the experimentally obtained transmittance data, except for the slight deviations at the initial photoexcitation and the relaxed quasi-equilibrium state. We also present the results similarly obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths.

The origin of superconductivity in the iron pnictides has been attributed to antiferromagnetic spin ordering that occurs in close combination with a structural transition, but there are also proposals that link superconductivity to orbital ordering. We used bulk-sensitive laser angle-resolved photoemission spectroscopy on BaFe(2)(As(0.65)P(0.35))(2) and Ba(0.6)K(0.4)Fe(2)As(2) to elucidate the role of orbital degrees of freedom on the electron-pairing mechanism. In strong contrast to previous studies, an orbital-independent superconducting gap magnitude was found for the hole Fermi surfaces. Our result is not expected from the superconductivity associated with spin fluctuations and nesting, but it could be better explained invoking magnetism-induced interorbital pairing, orbital fluctuations, or a combination of orbital and spin fluctuations. Regardless of the interpretation, our results impose severe constraints on theories of iron pnictides.

We have performed a temperature-dependent angle-integrated laser photoemission study of iron oxypnictide superconductors LaFeAsO:F and LaFePO:F exhibiting critical transition temperatures (Tc's) of 26 K and 5 K, respectively. We find that high-Tc LaFeAsO:F exhibits a temperature-dependent pseudogap-like feature extending over ~0.1 eV about the Fermi level at 250 K, whereas such a feature is absent in low-Tc LaFePO:F. We also find ~20-meV pseudogap-like features and signatures of superconducting gaps both in LaFeAsO:F and LaFePO:F. We discuss the possible origins of the unusual pseudogap-like features through comparison with the high-Tc cuprates.

We investigate the electronic reconstruction across the tetragonal-orthorhombic structural transition in FeSe by employing polarization-dependent angle-resolved photoemission spectroscopy on detwinned single crystals. Across the structural transition, the electronic structures around the $\mathrm{\ensuremath{\Gamma}}$ and $M$ points are modified from fourfold to twofold symmetry due to the lifting of degeneracy in ${d}_{xz}/{d}_{yz}$ orbitals. The ${d}_{xz}$ band shifts upward at the $\mathrm{\ensuremath{\Gamma}}$ point, while it moves downward at the $M$ point, suggesting that the electronic structure of orthorhombic FeSe is characterized by a momentum-dependent sign-changing orbital polarization. Due to this sign-changing orbital order, the elongated directions of the elliptical Fermi surfaces at the $\mathrm{\ensuremath{\Gamma}}$ and $M$ points are rotated by 90\ifmmode^\circ\else\textdegree\fi{} with respect to each other, which makes the nesting condition between these FSs imperfect. The present result, supported by calculations, indicates the possible suppression of the spin-fluctuation mediated superconductivity in the orthorhombic FeSe, as compared to the orbital-ordered state without sign change.

The topological aspects of electrons in solids emerge in realistic matters as represented by topological insulators. They are expected to show a variety of new magneto-electric phenomena, and especially the ones hosting superconductivity are strongly desired as the candidate for topological superconductors (TSC). Possible TSC materials have been mostly developed by introducing carriers into topological insulators, nevertheless, those exhibiting indisputable superconductivity free from inhomogeneity are very few. Here we report on the observation of topologically-protected surface states in a centrosymmetric layered superconductor, beta-PdBi2, by utilizing spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, several surface bands, some of which crossing the Fermi level, are clearly observed with symmetrically allowed in-plane spin-polarizations. These surface states are precisely evaluated to be topological, based on the Z2 invariant analysis in analogy to 3-dimensional strong topological insulators. beta-PdBi2 may offer a TSC realized without any carrier-doping or applying pressure, i.e. a solid stage to investigate the topological aspect in the superconducting condensate., Comment: 17 pages, 4 figures

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K2Cr8O16, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic- metal transition at T-C = 180 K and transforms into a ferromagnetic insulator below T-MI = 95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above T-C to T-MI, which effectively get pinned to an average nominal valence of Cr+3.75 (Cr4+:Cr3+ states in a 3:1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d-electrons/Cr) and the half-metallic ferromagnetism in the t(2g) up-spin band favor a low-energy Peierls metal-insulator transition.

We have investigated the low-energy electronic state of boron-doped diamond thin film by the laser-excited photoemission spectroscopy. A clear Fermi-edge is observed for samples doped above the semiconductor–metal boundary, together with the characteristic structures at 150 � n meV possibly due to the strong electron–lattice coupling effect. In addition, for the superconducting sample, we observed a shift of the leading edge below T c indicative of a superconducting gap opening. We discuss the electron–lattice coupling and the superconductivity in doped diamond. r 2006 NIMS and Elsevier Ltd. All rights reserved.

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

We report peculiar momentum-dependent anisotropy in the superconducting gap observed by angle-resolved photoemission spectroscopy in BaFe2(As(1-x)P(x))2 (x = 0.30, Tc = 30 K). Strongly anisotropic gap has been found only in the electron Fermi surface while the gap on the entire hole Fermi surfaces are nearly isotropic. These results are inconsistent with horizontal nodes but are consistent with modified s ± gap with nodal loops. We have shown that the complicated gap modulation can be theoretically reproduced by considering both spin and orbital fluctuations.

We study superconducting FeSe (Tc = 9 K) exhibiting the tetragonal-orthorhombic structural transition (Ts = 90 K) without any antiferromagnetic ordering, by utilizing angle-resolved photoemission spectroscopy. In the detwinned orthorhombic state, the energy position of the dyz orbital band at the Brillouin zone corner is 50 meV higher than that of dxz, indicating the orbital order similar to NaFeAs and BaFe2As2 families. Evidence of orbital order also appears in the hole bands at the Brillouin zone center. Precisely measured temperature dependence using strain-free samples shows that the onset of the orbital ordering (To) occurs very close to Ts, thus suggesting that the electronic nematicity above Ts is considerably weaker in FeSe compared to BaFe2As2 family., Comment: 6 pages, 4 figures

The nature of the pseudogap in high transition temperature (high-Tc) superconducting cuprates has been a major issue in condensed matter physics. It is still unclear whether the high-Tc superconductivity can be universally associated with the pseudogap formation. Here we provide direct evidence of the existence of the pseudogap phase via angle-resolved photoemission spectroscopy in another family of high-Tc superconductor, iron-pnictides. Our results reveal a composition dependent pseudogap formation in the multi-band electronic structure of BaFe2(As1-xPx)2. The pseudogap develops well above the magnetostructural transition for low x, persists above the nonmagnetic superconducting dome for optimal x and is destroyed for x ~ 0.6, thus showing a notable similarity with cuprates. In addition, the pseudogap formation is accompanied by inequivalent energy shifts in xz/yz orbitals of iron atoms, indicative of a peculiar iron orbital ordering which breaks the four-fold rotational symmetry., Comment: 14 pages, 10 figures

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe${}_{1\ensuremath{-}x}$Zn${}_{x}$)${}_{2}$As${}_{2}$ (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around $T\ensuremath{\sim}$ 135 K, indicating antiferromagnetic order below the N$\stackrel{\ifmmode \acute{}\else \'{}\fi{}}{\mathrm{e}}$el temperature of ${T}_{\mathrm{N}}\ensuremath{\sim}$ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below ${T}_{\mathrm{N}}$. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3$d$ state $\ensuremath{\sim}$10 eV below the Fermi level and do not participate in the formation of the FSs.

Charge dynamics and its critical behavior are investigated near the metal-insulator transition of layered-nickelate ${R}_{2\ensuremath{-}x}$Sr${}_{x}$NiO${}_{4}$ ($R=\mathrm{Nd}$, Eu). The polarized x-ray absorption spectroscopy experiment clearly shows the multiorbital nature which enables the ${x}^{2}\ensuremath{-}{y}^{2}$-orbital-based checkerboard-type charge ordering or correlation to persist up to the critical doping region ($x\ensuremath{\sim}1$). In the barely metallic region proximate to the charge-ordered insulating phase, the nominal carrier density estimated from the Hall coefficient markedly decreases in accord with development of the pseudogap structure in the optical conductivity spectrum, while the effective mass is least enhanced. The present findings combined with the results of recent angle-resolved photoemission spectroscopy show that the pseudogap in the metal-insulator critical state evolves due to the checkerboard-type charge correlation to extinguish the coherent-motion carriers in a characteristic momentum (k)-dependent manner with lowering temperature.

Many organic metals display exotic properties such as superconductivity, spin-charge separation and so on and have been described as quasi-one-dimensional Luttinger liquids. However, a genuine Fermi liquid behaviour with quasiparticles and Fermi surfaces have not been reported to date for any organic metal. Here, we report the experimental Fermi surface and band structure of an organic metal (BEDT-TTF)3Br(pBIB) obtained using angle-resolved photoelectron spectroscopy, and show its consistency with first-principles band structure calculations. Our results reveal a quasiparticle renormalization at low energy scales (effective mass m*=1.9 me) and ω2 dependence of the imaginary part of the self energy, limited by a kink at ~50 meV arising from coupling to molecular vibrations. The study unambiguously proves that (BEDT-TTF)3Br(pBIB) is a quasi-2D organic Fermi liquid with a Fermi surface consistent with Shubnikov-de Haas results., The physical properties of organic metals have generally been described in terms of a highly correlated Luttinger liquid. Using angle-resolved photoelectron spectroscopy, Kiss et al. measure the Fermi surface of (BEDT-TTF)3Br(pBIB), and find that, in contrast to other systems, it can be described as a Fermi liquid.

We investigate the two-dimensional (2D) highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X = I, Br, and Cl) by angular-resolved photoemission spectroscopy. Due to the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin-splitting. The characteristic 2D confinement effect is clearly observed also in the valence-bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX., Comment: 15 pages 4 figures

In layered polar semiconductor BiTeI, giant Rashba-type spin-split band dispersions show up due to the crystal structure asymmetry and the strong spin-orbit interaction. Here we investigate the 3-dimensional (3D) bulk band structures of BiTeI using the bulk-sensitive $h\nu$-dependent soft x-ray angle resolved photoemission spectroscopy (SX-ARPES). The obtained band structure is shown to be well reproducible by the first-principles calculations, with huge spin splittings of ${\sim}300$ meV at the conduction-band-minimum and valence-band-maximum located in the $k_z=\pi/c$ plane. It provides the first direct experimental evidence of the 3D Rashba-type spin splitting in a bulk compound., Comment: 9 pages, 4 figures

The three-dimensional Fermi-surface structure of hole-doped metallic layered nickelate Eu${}_{2\ensuremath{-}x}$Sr${}_{x}$NiO${}_{4}$ ($x=1.1$), an important counterpart to the isostructural superconducting cuprate La${}_{2\ensuremath{-}x}$Sr${}_{x}$CuO${}_{4}$, is investigated by energy-dependent soft-x-ray angle-resolved photoemission spectroscopy. In addition to a large cylindrical hole Fermi surface analogous to the cuprates, we observe a $\ensuremath{\Gamma}$-centered $3{z}^{2}\ensuremath{-}{r}^{2}$-derived small electron pocket. This finding demonstrates that in the layered nickelate the $3{z}^{2}\ensuremath{-}{r}^{2}$ band resides close to the ${x}^{2}\ensuremath{-}{y}^{2}$ one in energy. The resultant multiband feature with varying orbital character as revealed may strongly work against the emergence of the high-temperature superconductivity.

We report on laser-excited angle-resolved photoemission spectroscopy in the electron-doped cuprate ${\mathrm{Sm}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}}$. The data show the existence of a nodal hole-pocket Fermi surface both in the normal and superconducting states. We prove that its origin is long-range antiferromagnetism by an analysis of the coherence factors in the main and folded bands. This coexistence of long-range antiferrmagnetism and superconductivity implies that electron-doped cuprates are two-Fermi-surface superconductors. The measured superconducting gap in the nodal hole pocket is compatible with a $d$-wave symmetry.

Time-resolved photoelectron spectroscopy (trPES) can directly detect transient electronic structure, thus bringing out its promising potential to clarify nonequilibrium processes arising in condensed matters. Here we report the result of core-level trPES on 1$T$-TaS${}_{2}$, realized by developing a high-intensity 60-eV laser obtained by high-order harmonic generation. Ta$4f$ core-level trPES offers the transient amplitude of the charge-density wave (CDW), via the site-selective and real-time observation of Ta electrons. The present result indicates an ultrafast photoinduced melting and recovery of CDW amplitude, followed by a peculiar long-life oscillation (i.e., collective amplitudon excitation) accompanying the transfer of electrons among adjacent Ta atoms. core-level trPES offers a broad range of opportunities for investigating the ultrafast atom-specific electron dynamics in photorelated phenomena of interest.

We have investigated charge dynamics and electronic structures for single crystals of metallic layered nickelates, ${R}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4}$ ($R=\mathrm{Nd},\mathrm{Eu}$), isostructural to ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$. Angle-resolved photoemission spectroscopy on the barely metallic ${\mathrm{Eu}}_{0.9}{\mathrm{Sr}}_{1.1}{\mathrm{NiO}}_{4}$ ($R=\mathrm{Eu}$, $x=1.1$) has revealed a large hole surface of ${x}^{2}\ensuremath{-}{y}^{2}$ character with a high-energy pseudogap of the same symmetry and comparable magnitude with those of underdoped ($xl0.1$) cuprates, although the antiferromagnetic interactions are 1 order of magnitude smaller. This finding strongly indicates that the momentum-dependent pseudogap feature in the layered nickelate arises from the real-space charge correlation.

Electronic structures of the quantum critical superconductor $\ensuremath{\beta}\mathrm{\text{\ensuremath{-}}}{\mathrm{YbAlB}}_{4}$ and its polymorph $\ensuremath{\alpha}\mathrm{\text{\ensuremath{-}}}{\mathrm{YbAlB}}_{4}$ are investigated by using bulk-sensitive hard x-ray photoemission spectroscopy. From the Yb $3d$ core level spectra, the values of the Yb valence are estimated to be $\ensuremath{\sim}2.73$ and $\ensuremath{\sim}2.75$ for $\ensuremath{\alpha}$- and $\ensuremath{\beta}\mathrm{\text{\ensuremath{-}}}{\mathrm{YbAlB}}_{4}$, respectively, thus providing clear evidence for valence fluctuations. The valence band spectra of these compounds also show ${\mathrm{Yb}}^{2+}$ peaks at the Fermi level. These observations establish an unambiguous case of a strong mixed valence at quantum criticality for the first time among heavy fermion systems, calling for a novel scheme for a quantum critical model beyond the conventional Doniach picture in $\ensuremath{\beta}\mathrm{\text{\ensuremath{-}}}{\mathrm{YbAlB}}_{4}$.

We investigate the electronic structure of Chromium Nitride (CrN) across the first-order magneto-structural transition at T_N ~ 286 K. Resonant photoemission spectroscopy shows a gap in the 3d partial density of states at the Fermi level and an On-site Coulomb energy U ~ 4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high resolution (6 meV) laser photoemission reveals a clear Fermi edge indicating an antiferromagnetic metal below T_N. Hard x-ray Cr 2p core-level spectra show T-dependent changes across T_N which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. The electrical resistivity confirms an insulator above T_N (E_g ~ 70 meV) which becomes a disordered metal below T_N. The results indicate CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magneto-structural transition., Submitted to Physical Review Letters (February 2010) 11 pages, 3 figures in the main text, 1 Supplementary Information

Laser angle-resolved photoemission spectroscopy (ARPES) is employed to investigate the temperature (T) dependence of the electronic structure in BaFe2As2 across the magnetostructural transition at T{N} approximately 140 K. A drastic transformation in Fermi surface (FS) shape across T{N} is observed, as expected by first-principles band calculations. Polarization-dependent ARPES and band calculations consistently indicate that the observed FSs at k{z} approximately pi in the low-T antiferromagnetic state are dominated by the Fe3d{zx} orbital, leading to the twofold electronic structure. These results indicate that magnetostructural transition in BaFe2As2 accompanies orbital-dependent modifications in the electronic structure.

We report on the electronic structure of the perovskite oxide CaCrO3 using valence-band, core-level, and Cr 2p - 3d resonant photoemission spectroscopy (PES). Despite its antiferromagnetic order, a clear Fermi edge characteristic of a metal with dominant Cr 3d character is observed in the valence band spectrum. The Cr 3d single particle density of states are spread over 2 eV, with the photoemission spectral weight distributed in two peaks centered at ~ 1.2 eV and 0.2 eV below EF, suggestive of the coherent and incoherent states resulting from strong electron-electron correlations. Resonant PES across the Cr 2p - 3d threshold identifies a 'two-hole' correlation satellite and yields an on-site Coulomb energy U ~4.8 eV. The metallic DOS at EF is also reflected through the presence of a well-screened feature at low binding energy side of the Cr 2p core-level spectrum. X-ray absorption spectroscopy (XAS) at Cr L3,2 and O K edges exhibit small temperature dependent changes that point towards a small change in Cr-O hybridization. The multiplet splitting in Cr 2p core level spectrum as well as the spectral shape of the Cr XAS can be reproduced using cluster model calculations which favour a negative value for charge transfer energy between the Cr 3d and O 2p states. The overall results indicate that CaCrO3 is a strongly hybridized antiferromagnetic metal, lying in the regime intermediate to Mott-Hubbard and charge-transfer systems.

We have studied the momentum dependence of the energy gap of Bi2(Sr,R)2CuOy by angleresolved photoemission spectroscopy (ARPES), particularly focusing on the difference between R=La and Eu. By comparing the gap function and characteristic temperatures between the two sets of samples, we show that there exist three distinct energy scales, {\Delta}pg, {\Delta}sc0, and {\Delta}eff sc0, which correspond to T* (pseudogap temperature), Tonset (onset temperature of fluctuating superconductivity), and Tc (critical temperature of coherent superconductivity). The results not only support the existence of a pseudogap state below T* that competes with superconductivity but also the duality of competition and superconducting fluctuation at momenta around the antinode below Tonset., Comment: accepted in Phys. Rev. B

Low-energy electronic structure of optimally doped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is investigated using laser-excited angle-resolved photoemission spectroscopy. The surface state and the CuO chain band that usually overlap the ${\text{CuO}}_{2}$ plane derived bands are not detected, thus enabling a clear observation of the bulk superconducting state. The observed bilayer splitting of the Fermi surface is $\ensuremath{\sim}0.08\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ along $(0,0)\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})$ direction, significantly larger than ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$. The kink structure of the band dispersion reflecting the renormalization effect at $\ensuremath{\sim}60\text{ }\text{meV}$ shows up similarly as in other hole-doped cuprates. The momentum dependence of the superconducting gap shows ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave-like amplitude but exhibits a nonzero minimum of $\ensuremath{\sim}12\text{ }\text{meV}$ along the $(0,0)\ensuremath{-}(\ensuremath{\pi},\ensuremath{\pi})$ direction. Possible origins of such an unexpected ``nodeless'' gap behavior are discussed.

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+\delta}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($\Gamma$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $\Gamma \sim |\omega |$, while $\Gamma$($\omega =0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K $\lsim 0.07T_{\rm c}$), pronounced in the underdoped region.